Filling And Sanding Outside Walls

Applying paint on exterior walls will surprise you when it comes to the order of importance. The paint is nowhere near as big of a deal as the prep work. Sure, you can botch the paint job, but that’s fixable. Poor prep can equal catastrophic failure. 

Sanding and filling an outside wall is part of the necessary process in preparation for a paint job, whether you’re rolling it on or spray painting the entire thing. However, it’s also only part of the preparation process. 

As a matter of fact, sanding and filling will really constitute the vast majority of the legwork throughout the entire project. Surface prep is important for many reasons but the most important reason is to give the fresh paint something to bond to.

Without a proper bond, the paint will flake off, crack easily, or it can actually fall off in whole sections, which would leave you with no choice but to sand and strip the entire thing back down and start over. There are several steps to properly prep a surface.

  • Thoroughly clean and dry using mild detergent and water
  • Fill in (caulk) bad spots along the wall
  • Sand the entire surface area that you’re going to paint
  • Thoroughly clean and dry using mild detergent and water
  • Mask up
  • Paint

Filling The Bad Spots On An Exterior Wall

Depending on what type of exterior surface you’re going to paint, you’ll need some quality caulk to fill in your bad spots. If you don’t fill in your spots, it will stick out like a sore thumb under a fresh coat of paint. 

Also, spots that aren’t adequately filled will collect moisture, which will eventually flake and crack your paint. GE Sealants and Adhesive’s Max Shield is a good choice because it holds up well and applies to many surfaces.

GE advertises their caulk as being “paint ready” in as little as 30 minutes, however, give it an hour just to be safe. When you fill in cracks, divots, and damaged areas, it’s ok if the caulk isn’t completely flush with the surface, so long as it sticks out past the surface.

You want to fill in your bad spots enough to where there isn’t even the slightest depression to show that a crack or a hole used to be there. It’s ok if it sticks out a little from the crack or hole, so long as it isn’t over ¼” from the surface. 

Don’t worry about the fact that you can see the caulk bulging out slightly, that will be taken care of in the next step. 

Sanding The Surface Area Of An Outside Wall

Next comes the hard part. You can use a power sander if you want, however, if the material you’re dealing with is fragile, or if the sander eats into the surface easily enough to create depressions, you should fall back to hand sanding.

The last thing that you want is to lay on your fresh paint only to reveal all of your sanding marks because you got a little excited with a powered sander. It’s often safest to just hand sand it. 

  • Wear a dust mask
  • Start with 80-grit sandpaper
  • Sand in circular motions, covering every square inch
  • Make sure to remove all debris
  • Come back with 120-grit sandpaper
  • Sand light and quick, continuing circular motions
  • Feel the surface with your bare hands to ensure that it’s smooth

You want to work the sandpaper to clear off any marks and debris but you don’t want to work it so hard that it digs into the material. Remember, any new marks that you make will show up large and upfront when you apply fresh paint.

The 120-grit sandpaper is much more kind and will allow you to reach that smooth surface that you’re going for. You can tell that you’re there by sliding your bare hand over the surface, seeking anything that’s sticking out, and making sure it’s smooth.

Once all of your sanding is complete, make sure to thoroughly clean the surface. You don’t want to leave behind any dust, dirt, oil, or debris of any kind.

Why Filling And Sanding Is Necessary

The filling part is self-explanatory. Those divots and small depressions along the wall will stick out and be very obvious to even the casual viewer once you slap a fresh coat of paint on it. 

It’s very important to get those things filled and sanded back down until they’re flush with the surface. 

As far as sanding is concerned, the new paint needs something to bond to and when you open up the surface (the pores of the material) it allows the paint to bond with the surface at a molecular level.

This is especially true if you are spray painting. Residential spray painting is primarily done through conventional atomization. Industrial spray painting involves both conventional and airless atomization, depending on the contractor.

Regardless of the method, the paint is atomized at the tip of the spray gun, just before it leaves the gun and until it meets the surface. A well-sanded, and therefore open, surface allows the atomized paint to adhere to it better.

Once the paint cures, it’s not going anywhere unless it is impacted. Any sort of debris, dust, oil, or something else, resists the paint’s ability to properly adhere to the surface.

This makes those spots weak and the paint is only holding on by virtue of its connection to the whole paint job. In time, those weak spots will fail and, if it falls, it’s likely to pull a lot more paint down with it. 

All Things Considered

Without the proper prep work of sanding and filling an outside wall, the paint job is nothing more than a quick fix, potentially creating a larger problem in the near future. Paint simply won’t adhere properly to a surface that isn’t prepared specifically for it. 

If you’re thorough and patient, preparing and properly cleaning the surface beforehand, not only will your paint adhere permanently, it will also look and feel like a professional job.